"we stopped our cars. My client told me that it was absolutely necessary that I devise some excuse to stop the person whom I had seen go into the Gillett house when she started back toward the city. He said I simply must get in the car with her and remain with her until she reached a telephone booth and that I must learn the number she called. He said she was rather friendly and that I was to win her confidence. He said I was to spend the night with her, if possible."
"what did you do?"
(Mr. Carl Freeman Jasper and Mr. Hamilton Burger, taken from Chapter Fifteen, Erle Stanley Gardner "Perry Mason solves the case of the Bigamous Spouse")
Loosely, "even more so" or "with even stronger reason". Often used to lead from a less certain proposition to a more evident corollary. It is unwise to invest in pyramid schemes, and, a fortiori, in e-mail pyramid schemes.